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The Growth of Shrubs on High Arctic Tundra at Bylot Island: Impact on Snow Physical Properties and Permafrost Thermal Regime

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The Growth of Shrubs on High Arctic Tundra at Bylot Island: Impact on Snow Physical Properties and Permafrost Thermal Regime Biogeosciences Discuss., doi:10.5194/bg-2016-3, 2016 Manuscript under review for journal Biogeosciences Published: 4 February 2016 c Author(s) 2016. CC-BY 3.0 License. The growth of shrubs on high Arctic tundra at Bylot Island: impact on snow physical properties and permafrost thermal regime 5 Florent Domine1,2,3,4, Mathieu Barrere1,3,4,5,6, Samuel Morin5 1Takuvik Joint International Laboratory, Université Laval (Canada) and CNRS-INSU (France), Pavillon Alexandre Vachon, 1045 avenue de La Médecine, Québec, QC, G1V 0A6, Canada 2Department of Chemistry, Université Laval, Québec, QC, Canada 10 3Centre d’Etudes Nordiques, Université Laval, Québec, QC, Canada 4Department of Geography, Université Laval, Québec, QC, Canada 5Météo-France – CNRS, CNRM-GAME UMR 3589, CEN, Grenoble, France 6LGGE, CNRS-UJF, Grenoble, France 15 Correspondence to: F. Domine ([email protected]) Abstract. With climate warming, shrubs have been observed to grow on Arctic tundra. Their presence is known to increase snow height and expected to increase the thermal insulating effect of the snowpack. An important consequence would be the warming 20 of the ground, which will accelerate permafrost thaw, providing an important positive feedback to warming. At Bylot Island (73°N, 80°W) in the Canadian high Arctic where bushes of willows (Salix richardsonii) are growing, we have observed the snow stratigraphy and measured the vertical profiles of snow density, thermal conductivity and specific surface area (SSA) in over 20 sites of herb tundra and in willow bushes 20 to 40 cm high. We find that shrubs increase snow height, but only up to their own height. In shrubs, snow density, thermal conductivity and SSA are all significantly lower than on herb tundra. In shrubs, depth 25 hoar was observed to grow up to shrub height, while on herb, depth hoar developed only to 5 to 10 cm high. The thermal resistance of the snowpack was in general higher in shrubs than on herb tundra. More signs of melting were observed in shrubs, presumably because stems absorb radiation and provide hotspots that initiate melting. When melting was extensive, this increased thermal conductivity and reduced thermal resistance, countering the effect of shrubs in the absence of melting. Preliminary simulations of the effect of shrubs on snow properties and on the ground thermal regime were made with the Crocus snow 30 physics model and the ISBA land surface scheme driven by in-situ and reanalysis meteorological data. These predicted that the ground at 5 cm depth at Bylot Island during the 2014-2015 winter would be up to 13°C warmer in the presence of shrubs. 1 Biogeosciences Discuss., doi:10.5194/bg-2016-3, 2016 Manuscript under review for journal Biogeosciences Published: 4 February 2016 c Author(s) 2016. CC-BY 3.0 License. 35 1 Introduction Climate warming leads to shrub growth on Arctic tundra (Tape et al., 2006;Ropars and Boudreau, 2012). Shrubs are known to limit snow erosion by wind and to trap blowing snow, therefore increasing snow depth and perhaps snowpack duration (Liston et al., 2002;Lawrence and Swenson, 2011;Essery et al., 1999). Snowpack properties such as density, thermal conductivity and albedo are also known or suspected of being modified by shrubs (Liston et al., 2002;Ménard et al., 2014;Sturm et al., 2005). 40 These modifications have many potentially important consequences, including: (1) Snow physical properties affect air temperature and climate through the energy budget of the snow surface, which involves snow albedo and snow thermal conductivity; (2) Snow thermal properties (thermal conductivity, height, density) determine heat exchanges between the ground and the atmosphere in winter, and hence are a key factor in the thermal regime of permafrost. Permafrost thaw is recognized as an important positive climate feedback, because it could lead to the mineralization of organic matter stored there for millennia, 45 leading to the emission of poorly quantified but potentially enormous amounts of greenhouse gases (Koven et al., 2011). Illustrating the reality of this second consequence, Gouttevin et al. (2012) modelled the changes in permafrost thermal regime due to changes in snow thermal conductivity caused by the replacement of tundra by taiga and found ground temperature increases exceeding 10°C. While shrubs are not trees, this result does indicate the interest of investigating the impact of vegetation growth on snow physical properties. 50 Typical snowpacks on Arctic herb tundra first consist of a basal depth hoar layer which in general has a low density (200 to 280 kg m-3) and a low thermal conductivity (0.03 to 0.08 W m-1 K-1) (Domine et al., 2002;Domine et al., 2012;Sturm and Benson, 2004;Derksen et al., 2014). However, depth hoar can be indurated, i.e. formed by the metamorphism of a dense wind slab, and its density can then exceed 400 kg m-3 and its thermal conductivity 0.3 W m-1 K-1 (Domine et al., 2012;Sturm et al., 1997). Above that, one or several hard wind slabs made of small rounded grains are generally found (Domine et al., 2002;Domine et al., 55 2012;Sturm and Benson, 2004). These have densities from 300 to above 500 kg m-3 and thermal conductivities above 0.15 W m-1 K-1, reaching 0.6 W m-1 K-1 (Sturm et al., 1997).Other types of snow layers may also be observed above the basal depth hoar, such as layers of faceted crystals or other layers of depth hoar, sometimes located in between two wind slabs (Domine et al., 2002;Domine et al., 2012;Sturm and Benson, 2004). However a simplified and representative description of Arctic snowpacks on herb tundra is that they consist of a basal depth hoar layer of low density and low thermal conductivity, overlaid by a hard wind 60 slab of high density and high thermal conductivity. Several studies document some aspects of how snow properties are affected by shrubs. Liston et al. (2002) modelled the effect of shrubs growing on Arctic tundra on snow height and energy fluxes. They took into account changes in snow depth and thermal conductivity. These last changes were made by assuming that the depth hoar layer was thicker in shrubs. They assigned a depth hoar layer thickness and did not observe or simulate metamorphism and depth hoar growth in shrubs. 65 Euskirchen et al. (2009) modelled changes in vegetation under different climate evolution scenarios. They considered snow cover duration and its effect on albedo. They did not consider changes in snow properties. Lawrence and Swenson (2011) considered the effect of shrub expansion on active layer depth. They included some snow effects such as how shrubs affect snow redistribution by wind. One effect of shrubs on snow albedo is taken into account: its decrease due to stems protruding above the snow in spring. One of their conclusions was that blowing snow processes need to be incorporated in model projections of future 70 Arctic climate change. However, they did not consider changes in snow properties other than albedo and it is possible or even likely (Gouttevin et al., 2012) that their conclusions would be significantly modified if they had. Myers-Smith and Hik (2013) observed that on Arctic tundra, snow was much thicker in low shrubs than on herb tundra (typically 30 vs. 10 cm). They also measured that winter ground temperature was up to about 8°C higher (typically 4 to 5°C warmer) under 2 Biogeosciences Discuss., doi:10.5194/bg-2016-3, 2016 Manuscript under review for journal Biogeosciences Published: 4 February 2016 c Author(s) 2016. CC-BY 3.0 License. shrubs than under herb. They did not, however, observe snow properties. Ménard et al. (2014) performed a very detailed study of 75 shrub bending by snow and how this affected albedo. They focussed on tall shrubs and concluded that many other studies needed to be performed to better understand snow-shrubs interactions. These few investigations are very useful, but to our knowledge there is no extensive study of the impact of shrubs on snow physical properties. These are required to quantitatively understand and model the relationship between shrubs and ground temperature, especially in the context of shrub growth and expansion. Questions that deserve further or novel investigations 80 include: Is the depth hoar in shrubs different from that on herb tundra? To what height does depth hoar form in shrubs? What is the snow density profile in shrubs and how is this related to stem density? How does the profile of snow specific surface area (SSA) in shrubs differ from that on grass? Since SSA, together with density and impurity content, governs snow albedo and also the light absorption profile and therefore the radiative energy input to the snowpack, this is a critical question affecting metamorphism and all snow physical properties. How do shrub stems, which strongly absorb shortwave radiation, affect the 85 irradiance profile in snow? How do these light-absorbing stems increase the likelihood of snow melting events? This work attempts to contribute to some of these questions. We have performed field observations and measured snow properties at Bylot Island (73°N, 80°W), a high Arctic site where willow shrubs (Salix richardsonii) 10 to 40 cm high are growing on herb tundra featuring mosses and graminoids. Our measurements include snow height and vertical profiles of density, thermal conductivity and specific surface area. We compare data over grass and in shrubs and use our data to perform numerical 90 simulations intended to evaluate the impact of willow shrubs on the ground thermal regime. 2 Methods 2.1 Site selection Bylot Island is a high Arctic site (see map on Fig.
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